One question per week – questions related to stress corrosion

The brittle fracture of metal materials or metal structures caused by the combined effect of static tensile stress and a specific corrosive environment. 　　Conditions and characteristics of stress corrosion: (1) Tensile stress must be present ； (2) The environmental medium that constitutes a condition for stress corrosion to occur is specific ； (3) The stress corrosion cracking rate is much higher than that of other forms of local corrosion, but it is considerably lower than the rate of pure mechanical fracture ； (4) Stress corrosion fracture often occurs suddenly without any obvious signs, hence it is extremely hazardous ； (5) The crack patterns include intergranular, transgranular, and mixed types ； (6) Fracture morphology: macroscopically, it is a brittle fracture; microscopically, traces of plastic flow can still be observed on the fracture surface. 　　Ways to control stress corrosion cracking: (1) Reduce stress and eliminate residual stress ； (2) Use stress-corrosion resistant materials ； (3) Use cathodic protection ； (4) Add a corrosion inhibitor or remove harmful components from the medium.

The following are excerpts from materials found online: Fracture under the action of environmental media; Characteristics of corrosion fracture surfaces. In their operational life, actual metal components or parts often come into contact with various substances present in the surrounding environment. The effect of environmental media on the mechanical properties of metal materials is known as environmental effects. Due to environmental effects, metals can experience sudden brittle fracture even when the stress applied to them is below the material’s yield strength; this phenomenon is known as environmental fracture. Environmental fracture typically includes stress corrosion cracking (SCC), hydrogen embrittlement (HE), corrosion fatigue (CF), liquid metal embrittlement (LME), radiation embrittlement, etc. Section 1 Stress Corrosion Cracking 1. Characteristics of stress corrosion cracking Fracture that occurs in a material or component as a result of the combined effect of stress and a corrosive environment is called stress corrosion cracking. This type of fracture is the result of the combined effect of stress and corrosion, with these two factors reinforcing each other to accelerate material degradation and promote the early formation and propagation of cracks. When considering the effect of stress alone, it is found that the stress required to cause radial fracture is quite low; without the influence of environmental factors, such a low level of stress is absolutely safe. The danger of stress corrosion lies in the fact that it often occurs in relatively mild media and under low stress levels, which leads to it being frequently overlooked and results in a continuous occurrence of accidents. According to ASTM in the United States, losses caused solely by stress corrosion cracking there exceed 30 million dollars per year. Stress corrosion fracture has the following main characteristics: (1) Stress corrosion fracture is a brittle fracture, with a flat fracture surface that is perpendicular to the principal stresses. There was no significant plastic deformation before fracture, and the fracture surface had a granular shape. (2) Stress corrosion cracking is caused by static stress, which is much lower than the yield strength of the material, and it is generally a tensile stress (in recent years, it has also been found that in stainless steel, it can be caused by compressive stress). This tensile stress can be an applied stress or a residual stress. Residual stresses and microstructural changes resulting from welding and cold working can easily become causes of stress corrosion. (3) The environment for stress corrosion is specific; various media are only sensitive to certain materials. For example, α-brass will only corrode and be damaged in ammonia solution, whereas β-brass can crack in water ； Austenitic stainless steels with a face-centered cubic structure are prone to cracking in chloride solutions, a phenomenon commonly known as \"chloride embrittlement\", whereas ferritic stainless steels with a body-centered cubic structure are not sensitive to this effect ； \"Alkali embrittlement\" of low-carbon steel and low-alloy steel in caustic alkali solutions, and \"nitrate embrittlement\" in media containing nitrate ions” ； \"Ammonia embrittlement\" of copper alloys in an ammonia environment, and so on. (4) Stress corrosion cracks often develop numerous bifurcations and propagate continuously in a direction roughly perpendicular to the stress that drives their formation and growth; the propagation path can be transgranular, but it is usually intergranular. In the case of transgranular fracture, the fracture surface is cleaved or sub-cleaved, and the cracks exhibit V-shaped or feather-like patterns. (5) The crack growth rate in stress corrosion is generally between 10-9 and 10-6 m/s, which is a gradual and slow process; when this subcritical crack growth reaches a certain critical size, fracture occurs. (6) Cracks caused by stress corrosion generally originate from surface pits, and the propagation direction of these cracks is usually perpendicular to the tensile axis. (7) Stress corrosion is a type of localized corrosion, and the corrosion cracks are often covered by corrosion products, making them difficult to observe from the outside. (8) Pure metals do not suffer from stress corrosion; the impurity content and the content of alloying elements have a significant impact on stress corrosion. (9) Cathodic protection is highly effective in preventing stress corrosion cracking and stopping crack propagation. Table 9-1 Some environments in which stress corrosion occurs in metal materials
Material | Environmental medium
Carbon steel and low-alloy steel | Sodium hydroxide solution, nitrate solution, acidic hydrogen sulfide solution, seawater, marine or industrial atmospheres
Stainless steel | Acidic oxide solutions, sodium chloride-hydrogen peroxide solution, hydrogen sulfide, seawater, sodium hydroxide-hydrogen sulfide solution
High-strength steel | Rainwater, seawater, hydrogen sulfide solution, sodium chloride aqueous solution
Nickel-based alloys | Hot concentrated sodium hydroxide solution, hydrogen fluoride vapor and solutions
Aluminum alloys | Sodium chloride-hydrogen peroxide solution, sodium chloride aqueous solution, water vapor, seawater
Copper alloys | Ammonia vapor and solutions, aqueous solutions containing ammonium ions
Magnesium alloys | Sodium chloride-potassium chromate solution
Titanium alloys | Seawater, methanol, salt solutions
Monel | Hydrofluoric acid, fluorosilicic acid

The characteristics of stress corrosion fracture mentioned above can help us determine whether a fracture incident is due to stress corrosion. However, it is necessary to consider various factors as a whole; one should not draw conclusions simply based on a single characteristic. 2. Mechanism of stress corrosion The process of stress corrosion fracture also involves crack formation and growth, and can be divided into the following three stages: (1) Incubation stage: This is the period prior to crack formation; during this time, corrosion pits are formed to serve as the nuclei for cracks. When there are defects on the surface of the component that can act as sites for stress corrosion cracks (such as grain boundaries, twin boundaries, inclusions, etc.), there is no incubation stage – only a crack propagation stage. (2) Sub-stable crack propagation stage: Under the combined effect of stress and the environmental medium, the crack propagates slowly. (3) Crack instability propagation stage: Mechanical fracture that occurs once the crack reaches a critical size. There are various theories regarding the formation and propagation of cracks under the combined action of stress and environmental media. The more widely accepted theories to date include the anodic dissolution mechanism, in which anodic dissolution serves as the controlling process for fracture, and the hydrogen embrittlement mechanism, in which cathodic hydrogen absorption is the controlling process (the hydrogen embrittlement mechanism will be discussed later). It is important to note here the relationship between stress corrosion cracking (SCC) and hydrogen-induced cracking (HIC). Logically, they are “intersecting,” meaning that part of their content overlaps. If SCC is primarily caused by hydrogen evolution from the corroded cathode process, then this SCC is also HIC ； If SCC is primarily caused by the anodic dissolution process, then such SCC is not HIC. Under normal circumstances, cathodic polarization with an applied potential can be used to determine the SCC mechanism; that is, if accelerated fracture occurs, it belongs to the HIC mechanism ； If it slows down or is suppressed, it belongs to the anodic dissolution mechanism. This causes the anode potential to drop, accelerating the dissolution of the anode metal; cracks then gradually extend deeper, as shown in Figure 7-2. The aforementioned mechanism of anode dissolution actually involves four stages: sliding, film rupture, anode dissolution, and re-passivation. In the case of stress corrosion cracking that occurs through the grain, the breakdown of the protective film is caused by the formation of slip steps in local regions under stress. In the case of cracking that occurs along the grain boundaries, segregation at those boundaries or the precipitation of continuous phases leads to the formation of steps on the surface formed by individual grains, which in turn causes the breakdown of the surface protective film, ultimately resulting in stress corrosion cracking. For example, the stress corrosion cracking in martensitic stainless steel occurs mainly along grain boundaries, but when tempered below 455°C, it takes place through the grain boundaries. 3. Characteristics of stress corrosion fracture surfaces: The macroscopic morphology of stress corrosion fracture surfaces is quite similar to that of fatigue fracture surfaces, and it also consists of three zones: (1) the fracture initiation zone. It is generally caused by local corrosion or other types of cracks, such as pitting corrosion and crevice corrosion. These source cracks can be welding cracks, fatigue cracks, heat treatment cracks, etc. Stress corrosion microcracks originate from the surface and are discontinuous; they are characterized by numerous branches and a sharp, dendritic shape. (2) The metastable growth zone of stress corrosion cracks. This is a process of slow propagation of stress corrosion cracks, which is a result of the interaction between the material’s microstructure, stress, and environmental conditions. Macroscopically, this process is characterized by brittleness; even in Cr-Ni-based austenitic stainless steels with high plasticity, the cracks propagate along certain crystallographic directions of the material (such as cleavage planes), resulting in a black or grayish-black appearance. These corrosion products are quite important in the analysis of subsequent fracture incidents. (3) Final interruption zone. It is a rapid breakage zone or tearing zone, reflecting the properties of the matrix material. The microcracks in stress corrosion exhibit branching, indicating that during stress corrosion, a primary crack expands more rapidly while the other branch cracks expand more slowly. Based on this characteristic, stress corrosion can be distinguished from stress fatigue, intergranular corrosion, and other forms of fracture. The microstructure of stress corrosion fracture exhibits very distinct features: corrosion pits, corrosion products, and mud pattern textures. The mud pattern consists of linear cracks distributed on a flat surface (similar to the appearance of a dried riverbed); it is a coating formed by corrosion products. During transgranular fracture, the fracture surfaces observed under an electron microscope present as flat grooves (with a depth greater than the width), fan-shaped patterns, steps, and river-like patterns. The grooved areas are the result of the combined effect of stress and corrosive agents, while the fan-shaped patterns and steps arise from the connection of stress corrosion cracks on different planes; it is merely the difference in observation direction that results in varying fracture patterns. In short, the fracture characteristics of stress corrosion are quite complex. It is related to the crystal structure of the material, its mechanical properties, alloy composition, heat treatment condition, environmental atmosphere, as well as temperature and pressure conditions. It can exhibit a brittle fracture pattern, and a ductile fracture pattern can sometimes be observed as well; the mode of fracture can be intergranular or transgranular. For example, under normal conditions, low-carbon steel, low-alloy steel, aluminum alloys, and α-brass undergo grain-edge fracture, whereas β-brass and austenitic stainless steels exposed to chlorides generally experience transgranular fracture. Based on the aforementioned mechanisms and conditions that lead to stress corrosion, the main method of preventing stress corrosion cracking is to select materials appropriately; that is, materials with low sensitivity to stress corrosion should be chosen for components subjected to certain stresses and operating conditions. For example, brass is highly sensitive to stress corrosion cracking in the presence of ammonia, so copper alloys should be avoided as much as possible in components that come into contact with ammonia ； Secondly, reducing or eliminating residual tensile stress in the parts can lower their susceptibility to SCC; this is achieved by minimizing stress concentrations in the parts during design, ensuring uniform heating and cooling during manufacturing processes, and using annealing processes when necessary to eliminate stress. By counteracting or partially offsetting the effects of external tensile stress, it is possible to help prevent SCC ； Furthermore, improving the medium conditions can help prevent stress corrosion by adding corrosion inhibitors or protective coatings, as well as by reducing and eliminating harmful chemical ions that promote stress corrosion; stress corrosion can be avoided through such changes in the medium conditions ； For example, reducing the chloride ion content in cooling water and steam water through water purification is highly effective in preventing stress corrosion cracking in austenitic stainless steels; therefore, improving the design of metal components to prevent the accumulation of corrosive agents is an important measure for suppressing SCC. Finally, from the perspective of electrochemical protection, cathodic protection can also be used to prevent stress corrosion, as cathodic polarization can reduce the crack propagation rate; however, it should be noted that cathodic protection cannot be applied to high-strength steels or other materials sensitive to hydrogen embrittlement.

This post was last edited by brandon on 2009-9-12 at 12:45. 1. Corrosion that occurs in metals and alloys under the combined effect of corrosion and tensile stress is known as stress corrosion. Therefore, the cracking that occurs under these conditions is known as stress corrosion cracking, or SCC (Stress Corrosion Cracking). Stress corrosion occurs only in certain specific \"material-environment\" combinations, and it also requires the presence of tensile stress (external stress or residual stress resulting from welding, cold working, etc.). Cracking can occur even in environments with low corrosivity. There are generally two types of crack patterns in stress corrosion: one develops along the grain boundaries, known as intergranular fracture ； Another type is through the grains, known as transgranular fracture ； There are also mixed types, such as those where the main grain boundary crack is of the intergranular type, while the secondary cracks are of the transgranular type. Its cracks are perpendicular to the tensile stress. Since metal stress corrosion cracking is brittle and lacks obvious precursors, it is one of the most dangerous forms of corrosion. 2. The following media can easily cause stress corrosion cracking in austenitic stainless steel: hot chloride solutions, hot seawater, high-temperature water, hot NaCl solutions, trichloroethane, H2S aqueous solutions, NaOH aqueous solutions, concentrated boiler water, water vapor, and NaOH+sulfide aqueous solutions. In solutions containing ions susceptible to stress corrosion (such as Cl-, OH-, etc.), the stressed areas of austenitic stainless steel (such as those near welds) may experience dangerous stress corrosion cracking. In particular, solutions containing Cl- account for over 70% of the incidents causing stress corrosion cracking in austenitic stainless steels. The higher the Cl- concentration, the more likely cracking will occur, but there is no clear limit concentration below which cracking does not take place. There is an example in which cooling water containing Cl- accumulated in the gap between the tubes and the tube sheet of a chromium-nickel stainless steel heat exchanger, causing stress corrosion cracking of the stainless steel tubes, resulting in damage within 9 months. Therefore, even trace amounts of Cl- in water can be very dangerous for austenitic stainless steel. 3, 1) The water quality should be carefully assessed before selecting materials, in order to choose the appropriate ones. 2) Conduct reasonable design to avoid high-stress areas. 3) Establish a reasonable manufacturing process to eliminate residual stresses.

This post was last edited and published by lanye55 on 2009-9-16 08:38. A relatively comprehensive answer is provided here; all members who responded addressed the relevant questions. Now, a more detailed response to this issue is given: Stress corrosion cracking: It is a type of cracking that occurs in metals as a result of the combined action of stress (tensile stress) and corrosion, under certain temperature conditions. Stress corrosion is quite complex; in the absence of stress, corrosion is minimal ； Under stress, metals can crack even when corrosion is not severe. Since this cracking is brittle and occurs without any obvious warning signs, it can lead to catastrophic accidents. The main combinations of metal materials that can suffer from stress corrosion failure and their environments are as follows: 1. Carbon steel and its low-alloy steels: alkaline solutions, nitrates, anhydrous liquid ammonia, wet hydrogen sulfide, acetic acid, etc. 2. Austenitic stainless steels: chloride ions, chlorides + steam, wet hydrogen sulfide, alkaline solutions, etc. 3. Molybdenum-containing austenitic stainless steels: alkaline solutions, chloride aqueous solutions, sulfuric acid + copper sulfate aqueous solutions, etc. 4. Brasses: ammonia gas and its solutions, ferric chloride, wet sulfur dioxide, etc. 5. Titanium: methanol or ethanol containing hydrochloric acid, molten sodium chloride, etc. 6. Aluminum: wet hydrogen sulfide, hydrogen sulfide-containing substances, seawater, etc For stress corrosion environments related to metals, you can also refer to the table section on the second floor